It was a big sum for a 63-year-old battling cancer and living in Kansas City on less than $1,000 per month. But that single contribution — federal records show it was his first ever — quickly multiplied. Another $500 was withdrawn the next day, then $500 the next week and every week through mid-October, without his knowledge — until Mr. Blatt’s bank account had been depleted and frozen. When his utility and rent payments bounced, he called his brother, Russell, for help…

But what the Blatts believed was duplicity was actually an intentional scheme to boost revenues by the Trump campaign and the for-profit company that processed its online donations, WinRed. Facing a cash crunch and getting badly outspent by the Democrats, the campaign had begun last September to set up recurring donations by default for online donors, for every week until the election…

Contributors had to wade through a fine-print disclaimer and manually uncheck a box to opt out.

As the election neared, the Trump team made that disclaimer increasingly opaque, an investigation by The New York Times showed. It introduced a second prechecked box, known internally as a “money bomb,” that doubled a person’s contribution. Eventually its solicitations featured lines of text in bold and capital letters that overwhelmed the opt-out language.